The US Department of State has signed and issued the presidential permit for the construction of the controversial Keystone XL Pipeline, according to TransCanada Corp, the company behind the pipeline’s construction.

"We greatly appreciate President Trump's Administration for reviewing and approving this important initiative and we look forward to working with them as we continue to invest in and strengthen North America's energy infrastructure," TransCanada’s President and CEO Russ Girling said in a statement.

"This is a significant milestone for the Keystone XL project," he added.

President Donald Trump made the official announcement at the White House on Friday morning.

The pipeline which would link Canada’s heavy oil sands with refineries on the coast of Texas had been blocked multiple times throughout the permit process and was even blocked by former US President Barack Obama.

The former US president claimed the pipeline would not impact fuel prices for the average American and would greatly increase carbon emissions linked with climate change.

There is still a substantial amount of red tape for TransCanada to cut through before the pipeline can be completed as they will need further permits at a state level in Nebraska, Montana and South Dakota.

Environmental activists who opposed the pipeline are vowing to do so again.

“We’ve stopped the toxic Keystone XL Pipeline once, and we will do it again,” Dallas Goldtooth of the Indigenous Environmental Network said in a statement, adding that the Trump administration should “expect far greater resistance than ever before.”

The issuance of the permit has ended the company’s US constitutional challenge to Obama’s 2015 veto.

TransCanada had initially applied for a permit as far back as 2008. In one of his first acts in office, President Trump invited the company to resubmit the application.

The oil markets have been in a slump since the crash of 2014, in what many analysts have dubbed the ‘lower for longer’ era. Critics question the wisdom of building pipelines to carry oil fracked from North Dakota and Alberta shale deposits, in light of the widespread divestment initiatives in the oil industry taking place worldwide.